The user plane protocol stack at Layer 2 in the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) system consists of three sub-layers. They are, from high to low: Packet Data Convergence Protocol (PDCP) layer, Radio Link Control (RLC) layer and Media Access Control (MAC) layer. At the transmitting side, traffic is provided to a particular layer by receiving Service Data Units (SDUs) from a higher layer and Protocol Data Units (PDUs) are outputted to a lower layer. For example, the RLC layer receives packets from the PDCP layer. These packets are PDCP PDUs for the PDCP layer, but also RLC SDUs for the RLC layer. An inverse process occurs at the receiving side. That is, each layer sends SDUs to a higher layer, which receives them as PDUs. The PDCP SDUs are subjected to IP header compression, encryption and addition of PDCP headers and then mapped to PDCP PDUs. The RLC SDUs are segmented and concatenated according to a size specified at the MAC layer, added with RLC headers and then mapped to RLC PDUs. Each PDCP SDU is identified by a PDCP sequence number (SN). Each PDCP SDU has the same SN as its corresponding PDCP PDU and RLC SDU. Each RLC PDU is identified by an RLC SN.
In 3GPP LTE Release 11, each radio bearer has a PDCP entity and an RLC entity. Each Base Station (BS), or NodeB or evolved NodeB (eNB), and each User Equipment (UE) has a MAC entity. After successfully transmitting RLC PDUs corresponding to respective segments of an RLC SDU, the RLC entity in the BS or UE transmits to the PDCP entity in the same BS or UE an indication of successful transmission of the PDCP PDUs. Upon receiving the indication of successful transmission of the PDCP PDUs, the PDCP entity discards those PDCP PDUs and releases a retransmission buffer in the PDCP entity so as to receive more PDCP SDUs from the higher layer. Here, the UE can be a user terminal, a user node, a mobile terminal or a tablet computer.
The 3GPP LTE Release 12, which is currently being developed, involves standardization for dual connectivity enabled UE, master BS (master eNB) and secondary BS (secondary eNB). A master BS maintains Radio Resource Management (RRM) and measurement configurations for a UE, and requests from a secondary BS (the coverage of which is also referred to as “serving cell” of the UE) additional resources for the UE based on a received measurement report, a traffic condition or a bearer type. Upon receiving the request from the master BS, the secondary BS either configures a serving cell for the UE, or rejects the request due to lack of sufficient resources.
Based on different schemes for bearer split and different user plane protocol stacks, in 3GPP TSG-RAN2 Meeting 83bis, two user plane architectures, 1A and 3C, have been determined as standardization options for the dual connectivity deployment. As shown in FIG. 1, the option 3C has the following features: (1) the master BS communicates with a Serving Gateway (S-GW) via an S1-U interface; (2) the bearer split occurs in the master BS; and (3) for a split bearer, its corresponding RLC entity exists in both the master BS and the secondary BS. In the option 3C, the RLC entity at the secondary BS interacts with a higher layer (i.e., a PDCP entity at the master BS) via an X2 interface.
In a non-dual-connectivity deployment, since the PDCP entity and the RLC entity are located in one single BS, once an RLC SDU has been successfully transmitted, the RLC SDU and its corresponding PDCP SDU and PDCP PDU can be discarded. This process can be implemented inside the BS and no further standardization is needed. However, in the option 3C for dual connectivity deployment, the PDCP entity and one of the RLC entities corresponding to a split bearer is located in the master BS while the other one of the RLC entities is located in the secondary BS. The existing solutions for the non-dual-connectivity deployment cannot solve the problem of discarding a successfully transmitted PDCP PDU in the secondary BS and its corresponding PDCP PDU and PDCP SDU in the master BS. Thus, the PDCP SDU and the PDCP PDU that have been transmitted successfully will be stored in the retransmission buffer for a long time, until a discard timer associated with the PDCP SDU expires, resulting in a waste of storage space. Further, as the storage of the successfully transmitted PDCP SDUs and the PDCP PDUs in the retransmission buffer occupies a large storage space for a long time, when the higher layer transmits PDCP SDUs at a high rate while the PDCP retransmission buffer has been filled with PDCP SDUs that have been transmitted successfully but have not expired, the newly arrived PDCP SDUs will be dropped, thereby degrading the reliability of the radio link.